still have one questionthere are 50m long trees what then. i come to know and think of tat the pull is just not enough. try taking a longer straw. harder its to bring the water to the tap.the column tend to break.

What has not being taken into account is the primary reason why the stomata open and close. Stomata open and close to allow gas exchange for photosynthesis to occur. It lets carbon dioxide into the plant and oxygen out of the plant.

Carbon dioxide and oxygen, as well as water, does not passively diffuse into the plant.

Obviously, carbon dioxide is necessary for photosynthesis.

If the amount of oxygen increases within the plant (due to the splitting of water during photosynthesis) the photosynthetic rate decreases until it completely ceases through a process called photorespiration. This is due, in part, to the fact that RuBisCo (the enzyme responsible for the first step in carbonfixation) also fixes oxygen.

The opening and closing of the stomata to regulate carbon dioxide and oxygen levels within the plant allows for the "inadvertant" escape of water through diffusion. Under stressful conditions, drought, the plant will close stomata as an attempt to regulate water loss. But eventually, the plant has to open the stomata to regulate carbon dioxide/oxygen levels and additional water is lost. That, in part, is why plants have evolved different mechanisms (C3 versus C4 and CAM plants) to minimize water loss from the need to photosynthesize.

Under extremely high levels of humidity, the rate of transpiration decreases even though the stomata are open.

Although the opening and closing of the stomata may be an active process. Tranpiration itself is a passive process. The water is not being actively pumped out of the plant through the stomata.